Exposed RSJ on outside ?

Discussion in 'Building' started by LozM, 2 Apr 2021.

  1. LozM

    LozM

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    Hi,

    Have single storey extension at back of house and had planning approved for some renovation works. Part of that is to have 4.5m (3 panel) sliding doors at back in the single storey part. Had structural engineer design the beam needed and design says :

    250 x 150 x 80 RHS with 300mm 8mm thick bottom plate projection galvanised.

    I asked why galvanised (other beams being used elsewhere aren't) and he said because will be exposed on outside. I've had him explain this to me a few times now as just can't see why would be exposed on outside, is it not possible to design a beam which will be boxed in from inside and outside. I have tried googling pictures of this and just can't find any. He did say something about supporting the outer skin but when I see other houses with similar spans and most of those being two storey as opposed to our single I am not seeing any exposed beam.

    Will go back again but I'm just not getting it. He said could user hammerite galvanised paint to blend it in (house is white so are slider frames) but why is any of the beam exposed. Any help explaining greatly appreciated as must confess still confused.

    Many thanks,

    Lawrence
     
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  3. ^woody^

    ^woody^

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    He's probably technically correct and there may well be a standard somewhere requiring the steel to be protected.

    Usually, these are just primed, sometimes painted.
     
  4. tony1851

    tony1851

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    Assuming your cavity wall is 300 wide, it doesn't need a 300-wide plate. A 250-wide plate would be perfectly adequate, set in 30mm from the back of the RHS (which would actually make it easier to weld) and with the front edge of the plate 20 mm from the front face of the wall (a 20mm masonry overhang is perfectly acceptable).

    Where the beam runs onto the masonry each side of the opening, you obviously won't see the edge of the plate because it would be pointed over; just paint the whole edge and an inch or so inward with a couple of coats of Hammerite.

    Incidentally, a RHS is not cheap; if it's just supporting a roof and a layer or so of blockwork, a 152-deep or 203 deep section plus plate would be perfectly adequate and cheaper. Ask the SE why he hasn't suggested a cheaper section; if he mutters something about 'torsion', tell him to get his old college notes out:).
     
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  5. LozM

    LozM

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    Many thanks for detail posted, attached is engineers notes on the beam and does seem gone for a 300mm plate and the rendered wall is 300mm, wander if he means just plate will be showing or if one of the other measurements maybe the 250 when boxed in inside means beam will be showing outside, will attempt to get another explanation.

    Many thanks for taking time to reply.
     

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  6. denso13

    denso13

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    The underside of the welded plate will show on the outside leaf.

    [​IMG]
     
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  7. Notch7

    Notch7

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    Ive not seen RHS specified for a lintel before

    Every job I've done the engineer specs a universal beam -or column.

    in your case the engineer has specified a 150mm wide x 250mm high so as per the above pic, you will only see the edge of the plate in one of the mortar joints.
     
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  9. LozM

    LozM

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    A picture paints a thousand word, many thanks for that, will the edge of that plate show the whole way along on the outside ? Guess if that is the case then a very small area to paint and blend in, was worried would be a section of beam showing.

    Many thanks,

    Lawrence
     
  10. denso13

    denso13

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    Yes and the underside, depending how much your doors are set back from the outside leaf.


    [​IMG]
     
    Last edited: 3 Apr 2021
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  11. LozM

    LozM

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    Cool many thanks for that, I now understand having seen both pictures, was really struggling from engineers explanation.

    Cheers, much appreciated taken time to reply,

    Lawrence
     
  12. tony1851

    tony1851

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    @op;
    On Denso's pic, the plate is welded on the underside, but if you look carefully on the left, you'll see the plate is set in a little relative to the vertical wall of the RHS. This is because the fabricator obviously can't easily weld on the outside curved corner. If your SE has specified a 300 wide plate on the assumption that your wall is 300 thick (ie 100 block/cav/100 block), then the plate will stick out at the front from the face of the wall.

    Check with the fabricator, otherwise you might need to cut 20-25mm off the width of the plate for the full length. This is why a 250 wide plate is more practical.

    And as per Notch, beams of this type are usually made with open 'I' sections, which are also a lot easier for bolting the timber wall plate to the top. Also a lot easier to fix noggins for plasterboard/curtain rail etc.
     
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  13. LozM

    LozM

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    Cool, I can see why 300 would not be great as yes the blockwork is 300mm and if welded as shown will protrude for no real good reason, will speak to SE tomorrow and ask about that, many thanks as now understand and you will have saved me a piece of protruding plate.

    Many thanks,

    Lawrence
     
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